Gravitational wave radiation from close double white dwarfs in the Galaxy

Abstract

Close double white dwarfs (CDWDs) are believed to dominate the Galactic gravitational wave (GW) foreground radiation in the frequency range 10(-4) to 0.1 Hz, which will be detected by the Laser Interferometer Space Antenna (LISA). In this frequency range, the foreground radiation is predicted to be larger than both instrumental noise and the GW radiation from other sources, such as extragalactic low-mass X-ray binaries. We study the production of CDWDs using a binary population synthesis approach, and calculate their contribution to the Galactic GW foreground radiation. From a sample of 10(7) binaries, we select CDWD systems that would radiate at frequencies between 10(-8) and 10(-2) Hz. We calculate the evolutionary processes of the selected objects and conclude that the CDWD systems radiating at lower frequencies than 10(-3) Hz are LISA foreground sources, while those radiating at higher frequencies are possibly resolved sources. The CDWD systems which make up the LISA GW foreground are produced through three evolutionary channels: stable Roche lobe overflow (RLOF) plus common envelope (CE); CE plus CE; and exposed core plus CE, as proposed by Han (1998). We find that the resolved CDWDs are produced by the CE plus CE channel. We analyse the proportion of the resolved CDWDs that are compositionally double helium, or carbon-oxygen plus helium WD binary systems.